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MODERN PHYSICS TOPIC: MODERN PHYSICS General Objective: The
Learner should be able to use the nuclear and atomic models to
understand the production of X-rays and radioactivity. SUB-TOPIC:
Atomic and Nuclear structure. SPECIFIC OBJECTIVES: The Learner
should be able to;- • Describe the atom. • Define nuclides and
isotopes. • Represent nuclides with their atomic numbers and atomic
masses. • Give examples of isotopes. • Define nuclear fusion and
fission. • Balance equations of nuclear reactions. • Identify the
products of a nuclear reaction. • Explain the use of nuclear energy
in the generation of electricity and bombs. Modern physics deals
with the nuclear model of an atom. STRUCTURE OF AN ATOM According
to Neil Bohr and Rutherford an atom consists of a central nucleus,
in which the atom’s mass is concentrated, surrounded by electrons
that orbit round the nucleus. The simplest atom is that of
hydrogen. An atom consists of 3 particles namely -: Proton,
Neutrons and Electrons. The neutrons and protons are found in the
nucleus and are referred to as nuclei particles or nuclide
particles
Name Symbol Sign of change Protons H1
1 Positive Neutrons n0
1 No change
Electrons e−10 Negative
Protons are heavier than electrons. Protons are equivalent to a
positive hydrogen ion. ISOTOPES Isotopes are atoms of the same
element having the same atomic number but different mass numbers.
ATOMIC NUMBER Atomic number is the number of protons in the nucleus
of an atom. Symbol for atomic number is Z MASS NUMBER Mass number
is the sum of protons and neutrons in a nucleus of an atom. It is
sometimes called atomic mass. It is expressed using the letter A.
Note: Mass number = Atomic number + No. of Neutrons. A = Z + N An
atom is usually electrically neutral, implying that the number of
protons, Z is equal to its number of electrons. An atom X is
represented by : XZ
A Where A- mass number and Z – atomic number e.g. Cl17
35 . Has 17 protons and 18 neutrons
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QUESTIONS: 1. Given the atom : X27
59 , Find its (i) atomic mass (ii) atomic number (iii) number of
neutrons
(iv) number of electrons. 2. Describe the potassium atom
represented by the symbol K19
39 . SUB-TOPIC: RADIOACTIVITY SPECIFIC OBJECTIVES: The Learner
should be able to; • Define radioactivity. • Describe the nature of
alpha and beta particles and gamma rays. • List the properties of
the radioactivity. • Determine the effect of emissions on the
parent nucleus. • Define half-life to find the age and quantity
remaining. • State applications of radioactivity. RADIOACTIVITY
This is the spontaneous disintegration (breaking) of heavy unstable
nuclei to form stable nuclei with emission of radiations e.g beta
particles (β), gamma rays(γ), alpha particles (α). A RADIO ACTIVE
ELEMENT Is one whose nucleus spontaneously disintegrates and
continuously emits powerful and invisible radiations. DIFFERNCES
BETWEEN RADITIONS
Alpha (α) particle Beta (β) particle Gamma rays (γ) It is a
helium particle, He2
4 It is an electron, e−10 Are electromagnetic waves
Are positively charged Are negatively charged Have no charge.
Are less deflected by both magnetic and electric fields
Are more deflected by both magnetic and electric fields
Are not deflected by both magnetic and electric fields
BEHAVIOUR IN AN ELECTIC FIELD The alpha particles are deflected
towards the negative plate indicating that they are positively
charged. (Less deflected because they are heavy.) The beta
particles are deflected towards positive plate indicating that they
are negatively charged. (sharply deflected because they are very
light.) While gamma rays go through the field without being
deflected showing that the carry on charge.
_
+
α - particles
γ - rays
β - particles
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DEFLECTION BY A MAGNETIC FIELD
The beta particle is deflected down wards (north pole) because
they are negatively charged. They are sharply deflected because
they are very light. While alpha particles are deflected upwards
(South Pole) according to Flemings left hand rule because they are
positively charged. They are less deflected because they are heavy.
Gamma rays are not deflected because they possess no charge.
Magnetic field direction is into the paper. (ii) Magnetic field
direction is out of the paper PENETRATION OF MATTER: Alpha
particles have low penetrating power and are easily stopped by a
thin sheet of paper. They do not travel far in air because they are
easily slowed down by collisions with air molecules. Beta particles
are more penetrative than alpha particles but less penetrative than
gamma rays. They are stopped by thick paper, Perspex glass and thin
aluminum. Gamma rays possess the greatest penetrative power of the
three radiations. They are stopped by thick lead or concrete.
Travel in a straight line in air.
IONISATION OF AIR. Alpha particles have the highest ionizing
effect because they are heavy and carry a larger charge than beta
particles. Beta particles are less ionizing than alpha particles
because they possess a smaller charge and are very light. Gamma
rays are the poorest ionizers of the three radiations.
α ( 𝐻𝑒24 )
γ - rays
β ( 𝑒−10 )
β (Beta)
γ -rays
α (Alpha)
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TRACKS OF THE THREE RADTIONS AS DEMONSTRATED IN A CLOUD CHAMBER
When a radioactive source emits particles in an air space saturated
with water vapour or alcohol vapour inside a vessel with a glass
window, the speeding particles collide with the air molecules with
great force knocking off electrons, and leaving a trail of positive
and negative ions. If the air space is suddenly expanded by moving
the piston, cooling occurs and the vapour condenses out on the
ions, thus revealing the paths of the particles.
or
ALPHA PARTICLES Are short straight and bold tracks, this is
because they are good ionizers of gas. A large number of ions are
observed. The tracks differ in length due to difference in energy
BETA PARTICLES Tracks made by beta particles are longer and
fainter. They wonder as they are easily deflected by air molecules
because beta particles are light compared to the heavier air
molecules they collide with. GAMMA RAYS Gamma rays don’t leave on
actual track because they don’t ionize gas. If gamma rays are
present, whisky or wavy tracks. FLUORESCENCE: Only alpha particles
cause fluorescence when incident on a screen. NUCLEAR ENERGY
Nuclear energy is the type of energy made available from the
disintegration of the nucleus of an atom. 1. NUCLEAR FISSION
Nuclear fission is the splitting of nucleus of heavy atoms into
two lighter nuclei of roughly equal mass. The process Nuclear
fission can be started by the bombardment of a heavy unstable
nuclei with a neutron. The products of the process are two lighter
atoms and more neutrons which can make the process continue. The
two lighter products of nuclear fission are called fission products
or fission fragments.
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They have less mass than the correct value. The difference in
their mass is due to energy loss which is given by the Einstein
equation, E = mc2 where c is the speed of light and m is the mass
difference (or defect). The neutrons produced after nuclear fission
are called fission neutrons. Fission neutrons ensure the continuity
of nuclear fission indefinitely, resulting into a chain
reaction.
EXAMPLE OF NUCLEAR FISSION EQUATION:
ILLUSTRATION OF A CHAIN REACTION:
APPLICATION OF NUELEAR FISSION: • Used in making atomic bombs. •
Used to generate electricity. • Used to generate heat energy on
large scale. CONDITIONS NECESSARY FOR NUCLEAR FISSION TO OCCUR: •
There should be neutrons moving at a high speed that meet and are
captured by the heavy
nuclei to make it unstable. • There should be a heavy unstable
nucleus with isotopes which decay to produce isotopes
and high speed neutrons. 2. NUCLEAR FUSION
Nuclear fusion is the union of two light atomic nuclei to form a
heavy atom with the release of energy.
EXAMPLE OF NUCLEAR FUSION EQUATION.
CONDITIONS NECESSARY FOR NUCLEAR FUSION TO OCCUR • Temperatures
must be very high. • The light nuclei should be at very high speed
to overcome strong repulsive forces between
their charges. USES OF NUCLEAR FUSION: • Used to produce
hydrogen. • In the production of the Hydrogen bomb.
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• Used to produce electricity. • Used to produce heat energy on
large scale. Fusion reactions are sometimes known as thermonuclear
reactions because thermo energy has to be supplied before energy
can be released. NOTE: 1. The Sun produces its energy by nuclear
fusion. In the sun’s core, vast quantities of
energies are released as thermonuclear reactions convert
hydrogen into helium. 2. The hydrogen bomb is a result of an
uncontrollable fission chain reaction supplying
heat needed for the thermonuclear reaction to start. CHALLENGES
IN ACHIEVING CONTROLLED NUCLEAR FUSION: No ordinary container can
withstand the high temperatures required for nuclear fusion to
start and resist the expansion of the hydrogen so that the
reactions can be maintained. NUCLEAR EQUATIONS Alpha decay:
XZ A Y Z−2
A−4 + He + energy 24
Parent nuclide Daughter nuclide α - particle RULE 1: When an
element disintegrates (decays) by emission of an alpha particle, it
turns into an element two places earlier in the periodic table.
Beta decay:
XZ A Y Z+1
A + e + energy −10
Parent nuclide Daughter nuclide β - particle RULE 2: When an
element disintegrates (decays) by emission of a beta particle, it
turns into an element one place later in the periodic table. Gamma
Decay: Gamma rays are emitted as a result of instability in the
nucleus. Therefore, Gamma rays are emitted so that the nucleus
acquire a more stable state. The emission of Gamma rays causes no
change in the atomic and mass numbers of the element. EXAMPLES:
1. A radioactive substance X92238 undergoes decay and emits an
alpha particle to form Y.
Write down an equation for the process. SOLUTION An equation for
the process
238 =x + 4 ↔ x =234 92 = y + 2 ↔ y = 90
2. Unstable nuclei X88226 decays to form a stable nuclei Y and
beta particle is emitted.
Write down an equation for the process SOLUTION
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226 = n + 0 ↔ n =226 88 = m + - 1 ↔ m= 89
3. Radium Ra88
226 loses 5 α – particles and 4 β particles and is converted
into a new stable element, an isotope of lead Pb. Find the mass
number and atomic number of this isotope. SOLUTION
Ra88226 → Pb + 5(Z
A He) + 4(24 e)−1
0 226 = A +(5×4) +(4×0) = A + 20 Mass number of the isotope is,
A = 206 Also 88 = Z + (5 ×2) + (4 × -1) = Z +10 - 4 Atomic number
of the isotope is, Z = 82 4. Thorium Th90
232 is converted into Radium Ra88224 by radioactivity
transformation. How
many α and β emissions have taken place? SOLUTION
Th90232 → Ra + x(88
224 He) + y(24 e)−1
0 Change in Atomic Number: 90 = 88 + 2x − y y − 2x = 2
……………………………………..(1) Change in Mass Number: 232 = 224 + 4x x = 2 ,
therefore y = 2. There are 2 α – particles and 2 β – particles.
ARTIFICIAL RADIOACTIVITY: Artificial radioisotopes of some elements
can be prepared by bombarding nuclei of stable atoms with α -
particles, β – particles or neutrons. The process of producing
artificial radioactive nuclides is a reverse process of the decay
process – stable nuclei absorb nuclear particles of gamma photons
which strike them, and become unstable as a result. Activity is the
rate of disintegration or the number of disintegrations per second
of the radioactive substance. RADIO ISOTOPES: A radioisotope is an
unstable isotope produced by bombarding a stable nuclide with
either alpha, beta or neutrons. NOTE: Since radioisotopes are
unstable, they can decay with the emission of α-, β-, or γ
radiations to acquire a more stable state. EXAMPLES: When the
nucleus of Aluminium is bombarded by an α – particle, a radioactive
isotope of Phosphorus is obtained.
Al + He → P + n01
1530
24
1327
n + Na → Na 1124
11 23
01
Aluminium nucleus Alpha particle Phosphorous Isotope neutron
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n + I → I 53128
53 127
01
USES OF RADIOISOTOPES: - (SAME FOR APPLICATIONS OF
RADIOACTIVITY) Agricultural uses • Used in tracer techniques to
investigate the flow of liquids in chemical plants. • Used to
induce plant mutations to provide better seed varieties. Industrial
uses: • Used in the automatic control of thickness of material in
industries. • Study of wear and tear in machinery. (detecting
underground leakages in pipes) • Gamma ray are used to detect
faults in thickness of metals sheets in welded joints • Used in
packaging process by counting the correct amount or number per
packet. Medical uses • Used in treatment of cancer. • They are used
to kill bacteria in food (x- rays) • Used to sterilize medical
equipment like syringes • Used in the diagnosis and treatment of
goiter. CARBON DATING: This is the estimation of age of a substance
by studying the count rate of a radioactive sample in the
substance. Archeology Used to determine the time that has elapsed
since death of organisms occurred, a process called carbon dating.
Explanation Living plants absorb and contain a radioactive isotope
of carbon having a half-life of about 5600 years. As long as the
plant lives, the count rate of this isotope is constant. When a
plant dies, it stops absorbing the carbon isotope, but
radioactivity continues. So, the count rate falls accordingly. By
determining the count rate of wood, its age can be estimated. The
same procedure can be used to determine the age of dead fossils.
Geology They are used to determine the age of rocks. When rocks
were formed, some radioisotopes were trapped in them. By the number
of the radioisotopes (parent nuclides) remaining in a rock sample
with the daughter nuclides, the age of the rock can be determined.
DANGER (HAZARDS) OF RADIATIONS • Beta and alpha particles cause
skin burns and sores. • Can cause cancer and affect eye sight. •
May cause infertility and sterility, (reproductive organs and
liver). • May lead to genetic mutations (abnormalities).
neutron Normal stable sodium nuclide Radioisotope of sodium used
in medicine
neutron stable iodine nuclide Radioisotope of iodine used in
medicine
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SAFETY PRECAUTIONS WHEN DEALING WITH RADIOACTIVE SOURCES •
Radioactive sources must be kept in lead boxes • Handle radioactive
materials using tweezers. • Workers should wear protective lead
suits (protective clothing) • Walls of industries are made of thick
strong concrete to prevent exposure to surroundings. • Using
radioactive materials of short half – life • Washing body
thoroughly after exposure to radioactive materials. • Avoid eating
or drinking around radioactive sources. Back ground radiation These
are radiations which naturally exist even in the absence of
radioactive source. They are caused by natural tracks of
radioactive materials in rocks, in air, Cosmic rays from outer
space as well as bricks of buildings. Cosmic rays are very high
energetic radioactive particles which come from deep in space. So
the correct count = actual rate - back ground count rate. E.g.
Given that the back ground rate is 2 counts per minute and the
Geiger Muller count rate is 25 counts per minute, determine the
approximate number of radiations present. Count rate = 25 - 2 =
23c/min HALF LIFE: The half – life of a radioactive substance, t is
the time taken for the radioactive substance to decay to half of
its original mass. EXAMPLE 1 If a radioactive element of mass 32
decays to 2g in 96 days. Calculate the half life. METHOD 1
4t = 96 ∴ t =2 4 days, is the half – life. METHOD 2: TABLE
FORM
No. of half – lives Time taken Amount present 0 0 32g 1 T 16g 2
2t 8g 3 3t 4g 4 4t 2g
Where t = the half-life therefore, 4t = 96 days
t =96
4= 24 days.
EXAMPLE 2: A certain radioactive substance takes 120years to
decay from 2g to 0.125g. find the half life Let the half –life be
t. METHOD 1.
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4 t =120 ⇔ t =30 years METHOD 2: TABLE FORM
No. of half - lives Time taken Amount present 0 0 2g 1 T 1g 2 2t
0.5g 3 3t 0.25g 4 4t 0.125g
Where t = the half-life therefore, 4t = 120 days
t =120
4= 30 days.
EXAMPLE 3: The half life of substance is 5 days. find how long
it takes for its mass to disintegrate from 64g to 2g
5 x 5 =25 days EXAMPLE 4: A radioactive element has a half life
of 4 years. if after 24years. 0.15g remains. Calculate the initial
mass of the radioactive material
Mo = 9.6g - check your notes girl. EXAMPLE 5: A certain mass of
a radioactive material contains 2.7 x 1024 atoms, how many atoms
decayed after 3200years if the half life of material is
1600years
Mass remaining = 6.75 x 1023 atom Mass decays = original mass -
mass remaining = (2.7 x 1024 - 6.75 x 1023) = 2.025 x 1024 atoms
GRAPHICAL METHOD OF DETERMINING HALF LIFE When a graph of account
rate against time or radioactive nuclei is drawn, the half life of
the radioactive nuclei can be determined as below.
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Examples 1. The following values obtained from the readings of a
rate meter from a radioactive
isotope of iodine. Time (min) 0 5 10 15 20 Count rate (Min -1)
295 158 86 47 25
Plot a suitable graph and find the half-life of the radioactive
iodine. 2. The following figures were obtained from Geiger Muller
counter due to ignition if the
sample of radon gas Time\min 0 102 155 ……. 300 Rate \min-1 1600
200 100 50
(a) (i) plot a graph of count rate against time (ii) Determine
the half life (iii) Find the missing values (b) (i) What is the
count rate after 200 minutes (ii) After how many minutes is the
count rate 1000 minutes NOTE: In general, if No is the initial
number of atoms, then after n half-lives the number remaining is
given by:
Number remaining/amount remaing = 𝑰𝒏𝒊𝒕𝒊𝒂𝒍 𝒂𝒎𝒐𝒖𝒏𝒕
𝟐𝒏
Thus, the fraction remaining after n half-lives is 𝐟 = 𝟏
𝟐𝐧
𝐓𝐡𝐞𝐧,𝐀𝐦𝐨𝐮𝐧𝐭 𝐫𝐞𝐦𝐚𝐢𝐧𝐢𝐧𝐠
𝐎𝐫𝐢𝐠𝐢𝐧𝐚𝐥 𝐚𝐦𝐨𝐮𝐧𝐭 =
𝟏
𝟐𝐧
1. Given 20 g of a radioactive sample of half-life 12 minutes,
how much of it remains after
36 minutes Solution
Number of half-lives, n = 36
12 = 3
Amount remaining
Original amount =
1
23 =
1
8
Amount remaining = 1
8 x 20 = 2.5 g
2. The activity of a radioactive source decreases from 1000
counts per minute to 125
counts per minute in 42 minutes. What is the half-life? Solution
Let n = number of half-lives
Then, 2n = 1000
125 = 8
n = 3
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So, 42 minute = 3 half-lives Half-life = 14 minutes
3. A radioactive sample has half-life of 2500 years. How long
does it take for three-
quarters of the sample to decay? Solution
Fraction remaining = 1
4 =
1
2n
number of half-lives that elapsed, n = 2 So, this will be after
2 x 2500 = 5000 years.
REVIEW QNS ON RADIOACTIVITY (HOW TO PASS PHYSICS PAGE 272)
SUB-TOPIC: Electrons SPECIFIC OBJECTIVES: The Learner should be
able to;
• Define thermionic emission and cathode rays. • Describe the
experiment to produce cathode rays. • Investigate the properties of
cathode rays. • List the uses of cathode rays. • Draw the CRO and
explain how it works. • Draw wave forms produced on a CRO. •
Mention uses of CRO.
THERMIONIC EMISSION
This is the process by which electrons are emitted from a hot
metal surface. The emitted electrons are called thermions.
EXPLANATION OF THERMIONIC EMISSION. When a metal is heated to a
certain temperature, some of its electrons gain sufficient energy
to overcome the electrostatic attractive forces and break free from
the metal surface and escape into the surrounding space. NOTE:
Thermionic emission increases with temperature.
CATHODE RAYS Cathode rays are streams of fast moving electrons.
PRODUCTION OF CATHODE RAYS The circuit is connected as shown The
cathode is a tungsten filament heated by a low a.c. voltage of
about 6.0 V such that it emits electrons by method of thermionic
emission.
Beam of electrons
Fluorescent screen
Evacuated glass tube
Anode with a hole Hot Filament cathode
3.5 kV (E.H.T)
6.0 V (a.c.)
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The large p.d of about 3.5 kV across the anode accelerates the
electrons from cathode towards the anode. The fast moving electrons
pass through the anode and strike the fluorescent screen such it
glows. The glass tube is evacuated to ensure that electrons move
freely so that they don’t collide with the relatively heavier air
molecules.
PROPERTIES OF CATHODE RAYS • They carry a negative charge (since
they are fast moving electrons). • They are deflected by both
electric and magnetic fields. • They ionize gases. • They cause
some substance to fluorescence i.e. give off light when they strike
the surface. • They travel in a straight line. • In an electric
field, cathode rays are deflected towards the positive plate and in
the
magnetic field, the direction of deflection is determined using
Flemings left hand rule. • They possess energy. • They can cause
certain metals to produce X – rays when they are incident on them.
Deflection of cathode rays by an Electric field.
Cathode rays are deflected toward the positive plate by an
electric field. Deflection of cathode rays by a Magnetic field.
or
In a magnetic field, the deflection of the cathode rays is
determined using Flemming’s Left hand rule. N.B. Cathode rays flow
in the opposite direction of conventional current.
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EXPERIMENT TO SHOW THAT CATHODE RAYS TRAVEL IN STRAIGHT LINE
(THERMIONIC TUBE) or
Cathode rays are incident on the maltase cross. A shadow of the
cross is formed on the fluorescent screen. The formation of the
shadow verifies that cathode rays travels in a straight line.
Showing that Cathode Rays Convey Negative Charge Cathode rays are
directed to enter Faraday’s cage in the tube which is connected to
a negatively charged gold leaf electroscope. Further divergence of
the leaf confirms negative charge. THE CATHODE RAY OSCILLOSCOPE
(C.R.O) Thermionic emission is utilized in the: • cathode ray
oscilloscope (C.R.O) • X –ray tube, • TV etc
The C.R.O consists of three main components. THE ELECTRON GUN
The electron gun consists of the following parts
Heater Faraday’s cage
Anode
Already negatively charged
Cathode Vacuum
- + High voltage
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(a) The cathode, C – used to emit electrons by thermionic
emission. (b)The control grid, G – this is connected to low voltage
supply and is used to control the number of electrons passing
through its central hole from the cathode to the anode. It acts as
the brightness control. (c)The anode – it accelerates the electrons
and also focuses them on to the screen. N.B: Since the grid
controls the number of electrons moving towards the anode. It
consequently controls the brightness of the spot on the screen.
DEFLECTING SYSTEM This consists of the X and Y plates. They are
used to deflect the electron beam horizontally and vertically
respectively. FLUORESCENT SCREEN. This is where the electrons beam
is focused to form a bright spot. How the cathode ray oscilloscope
works? The cathode is heated by a low voltage power supply. The
cathode emits electrons by thermionic emission. The electrons are
attracted and accelerated by the anode and focused onto the screen.
The grid controls the brightness of the spot on the screen.
DEFLECTING SYSTEM Beyond anode are two pairs of deflecting plates
to which p.d.s can be applied. The Y-plates are horizontal but
create a vertical electric field which deflects the beam
vertically. The X-plates are vertical and deflect the beam of
electrons horizontally. FLUORESCENT SCREEN It produces a spot of
light when electrons hit the screen. The Time base. This is a
special circuit connected to the X-plates for the purpose of
controlling the horizontal movement of the spot. Below are examples
of the display on the screen: This is the circuit connected to the
X – plates and is used to move the bright spot on the screen
horizontally. The time base uses a voltage referred to as a saw –
tooth voltage. The time- base is an electrical circuit which
generates a saw-tooth type of voltage shown below:
(iii) X-plate sweep and Y-
plate signal combined
(i) X-plate sweep switched on. (no signal on Y-plates)
(ii) a.c signal applied to Y-plates with X-plates switched
off
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When the voltage rises, the spot moves to the right with uniform
speed and then quickly flies back to the original spot as the
voltage drops down. The process is rapidly repeated to result into
a straight horizontal line across the screen. The appearance of the
horizontal line on the screen with Time base on X plate only.
Different wave forms. When time base (x- plate) is switched on and
there is no signal on the y-plate, the spot is deflected
horizontally. The horizontal line is observed. When alternating
current (a.c) is applied to the y- plate and time base (X –plate)
is off, the spot is deflected vertically . The vertical line
observed. When a.c is applied on the Y-plate and X- plate is on ,a
wave form is observed on the screen . When time base is switched
off and no signal to the Y- plate, a spot is observed.
(d) both time base and Y – plate switched off
Direct current applied to the Y – plate. USES OF A C.R O 1.
Measurement of p.d (voltage) A C.R.O can be used as voltmeter
because the distance through which the spot is deflected depends on
the p.d between the plates. Method The sensitivity of the Y-scale
is set to a particular voltage per division for example 5Vdiv−1 or
5Vcm−1. The unknown voltage is then applied to the Y-plates and the
distance moved by the spot from the mean position is noted. The
voltage is then calculated using the formula:
Peak voltage = Y − sensitivity × number of divisions
+5𝑉
0 𝑉
−5𝑉
0 𝑉
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Examples (ref: Password – Numerical problems in Physics pages
230-231) 1. The sensitivity of the Y-scale is set to 3 V cm−1. When
a voltage is applied to the Y-
plates, the spot on the screen moves 4cm from the mean position.
Calculate the peak voltage applied. (Ans= 12V)
Peak voltage = Y − sensitivity × number of divisions = 3 × 4 =
12V
2. The figure above shows a waveform on a screen of a cathode
ray oscilloscope. The Y-sensitivity is set at 2.5 V div−1.
Determine;
(a) the peak voltage Peak voltage = Y − sensitivity × number of
divisions
= 2.5 × 2 = 5V
(b) the peak-to-peak voltage Peak − to − peak voltage = 5 × 2 =
10V 2. Frequency measurements This is achieved by comparing a wave
form of known frequency with unknown frequency. Method The time
base control of the CRO is set to a particular value. The a.c
signal of the unknown frequency is applied to the Y-plates. The
number of divisions for one complete wave is noted. Time for one
wave is calculated, which is the period, T. Frequency 𝑓, is then
calculated using
the formula: 𝑓 =1
𝑇
Examples (ref: Password – Numerical problems in Physics pages
232-233 )
1. In the figure shown, the time-base control is set at 20 ms
cm−1. Calculate the frequency
of the wave shown. (Ans = 12.5Hz). For 1 complete wave, number
of divisions = 4
1cm = 20ms 4cm = 20 × 4ms = 80ms
f =1
𝑇=
1
0.08
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= 12.5Hz
2. The Y-gain and the time base controls of a CRO are set at
200mV cm−1. And 100ms cm−1 respectively. When an a.c signal is fed
to the Y-plates, the waveform displayed is as shown in the figure.
Determine the peak voltage and the frequency of the signal.
Peak voltage = Y − sensitivity × number of divisions = 3 × 200 =
600mV
= 0.6V
Time for 1 wave = 8 divisions (cm) 1division (cm) = 10ms
Period, T = 8 × 10 = 80ms = 0.08s
f =1
𝑇=
1
0.08
= 12.5Hz 3. Used to study wave forms of current and voltage. 4.
Used in manufacture of T.V. Summarised Uses of C.R.O 1. Measure
voltage. 2. Measure frequency. 3. Measure phase difference. 4.
Measure small time interval. 5. Used in manufacture of T.V. 6. Used
to study wave forms of current and voltage. REVIEW QNS ON CATHODE
RAYS (HOW TO PASS PHYSICS PAGE 237) AND PASSWORD – NUMERICAL
PROBLEMS IN PHYSICS PAGES 233-239 SUB-TOPIC: X-rays SPECIFIC
OBJECTIVES: The Learner should be able to; • Draw the structure of
the X-ray tube and describe how X-rays are produced. • List
properties and uses of X-rays. • State health hazards of X-rays and
safely precaution.
X – RAYS X – rays are electromagnetic radiations produced when
fast moving electrons are stopped by a metal target.
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TYPES OF X – RAYS There are two types of X – rays, namely (i)
Soft X- rays (ii) Hard X – rays Soft X –rays are produced at low
voltages. They have a low penetrating power i.e low frequency and
long wave length. Hard X –rays are produced at high voltages. They
have a high penetrating power i.e very high frequency and short
wave length. X – RAY PRODUCTION Production of x-rays. The cathode
is heated to emit electrons by thermionic emission using a low
voltage supply. A large p.d is used to accelerate the electrons
towards the anode along a highly evacuated tube. On reaching the
anode, they hit the metal target of a high melting point and their
kinetic energy is converted into heat and X- rays. The heat
generated around the anode is conducted away through the copper
anode to the cooling (radiator) fins. N.B: The X– ray tube is
evacuated to so that electrons can move freely with out any
hindrance from the air molecules. The target is a metal of high
melting point like tungsten so that it does not melt as a result of
the great amount of heat generated. The Anode is of copper which
rapidly conducts away the heat to the cooling fins. The fins are
painted black to quickly radiate heat to the surroundings.
PROPERTIES OF X- RAYS • X-rays readily penetrate through matter. •
They are not affected by electric and magnetic fields (since they
carry no charge). • They have no charge. • They cause ionization. •
They travel in straight lines. • They affect photographic material
(-by blackening it). • They cause certain materials to
fluorescence. • They are electromagnetic waves and travel at the
speed of light. USES OF X- RAYS (a) Medicine In medicine X – rays
are used to; • Investigate born fractures.
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• Detect lung tuberculosis. • Treat cancer especially when it
hasn’t spread by radiotherapy i.e very hard x-rays are
directed to the cancer cells so that the latter are destroyed •
Detect internal ulcers along a digestive track • Locate foreign in
the body e.g. swallowed metal objects (b) Industrial use In
Industries, X – rays are used to; • Detect cracks in car engines
and und8erground pipes. • Locate internal imperfections in welded
joints e.g pipes, boilers,storage tanks e.t.c. • Detect cracks in
building.
(c) X-ray crystallography
Used to determine inter – atomic spacing in the crystal.
Differences between cathode rays and X- rays.
Cathode rays X- rays Are negatively charged Have no charge Are
fast moving electrons Are electromagnetic waves Are deflected by
both magnetic and electric fields
Are not deflected by both magnetic and electric fields
HOW AN X-RAY IS USED TO LOCATE BROKEN PARTS OF A BONE. Bones are
composed of much denser material than flesh hence, if X- rays are
passed through the body, they are absorbed by the bones onto a
photographic plate which produces a shadow photograph of the bones.
Differences between Gamma rays and X- rays.
Gamma rays X- rays They are produced by unstable radioactive
material.
They are produced when fast moving electrons are stopped by a
metal target.
SIMILARITIES BETWEEN X - RAYS AND GAMMA RAYS: • They are both
electromagnetic waves. • They carry no charge. • They are not
deflected by both magnetic and electric fields. • They penetrate
matter. • They cause fluorescence. • They can cause harmful
effects. • They travel at the speed of light and in a straight
line. HARMFUL EFFECTS OF X-RAYS: • Hard X -rays destroy healthy
body cells. • They cause genetic mutation or changes. • They cause
damage of eye sight and cause blood cancer. • They produce skin
burns. PRECAUTIONS FOR SAFETY • Avoid unnecessary exposure to X
–rays. • Keep exposure time as short as possible. • The X- ray beam
should only be restricted to parts of the body being investigated.
• Workers dealing with X-rays should wear shielding jackets with a
layer of lead.
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• Exposure should be avoided for unborn babies and very young
children. • Rooms where X- ray machines are located (e.g. hospitals
and industries) are made of thick
concrete walls to absorb stray radiations. REVIEW QNS ON X -
RAYS (HOW TO PASS PHYSICS PAGE 249) PHOTO ELECTRIC EMISSION This is
the emission of electrons from a certain metal plate e.g zinc
plate, when electromagnetic radiations of short wave length fall on
it.
PHOTOELECTRONS: Photoelectrons are the electrons emitted by a
metal by the process of photoelectric effect. Photoelectrons are
emitted from any metal if the wavelength of incident
electromagnetic radiation is below a certain critical value called
the threshold wavelength. OR if the frequency of the incident
electromagnetic radiation is above the critical threshold
frequency) WORK FUNCTION: The Work function is the minimum
frequency of the incident radiation required to eject a
photoelectron from a particular metal surface. The number of
photoelectrons emitted from the metal surface depends on; (i) the
intensity of the incident radiation. Increasing intensity. (ii)
increases the number of electrons emitted. (iii) the type of
metal.
The incident radiation provides sufficient energy to overcome
the binding forces of the metal and the excess energy is converted
to into kinetic energy which the electrons use to escape from the
metal surface.
THE PHOTOELECTRIC CELL. The photoelectric cell uses
photoelectric effect to convert light energy into electric energy.
The strength of the current produced depends on the intensity of
the incident light radiation on the metal.
When a suitable radiation falls on the zinc cathode, it emits
electrons by photoelectric emission.
Evacuated transparent tube
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The anode attracts the electrons which then pass through an
external circuit causing an electric current. N.B: If gas is
introduced into the tube, the current decreases slowly because the
gas particles collide with the electrons, hence reducing the number
of electrons reaching the anode. APPLICATIONS OF PHOTOELECTRIC
EFECT: Photoelectric effect is applied in: • Burglar alarms. •
Automatic lighting systems • In solar calculators. • Television
cameras • Automatic door systems • Sound track on a film.
EXPERIMENT TO DEMONSTRATE PHOTOELECTRIC EFFECT: When Ultra violet
light is incident on a clean zinc plate placed on the cap of a gold
leaf electroscope: • If the electroscope is uncharged, the leaf
initially rises indicating that is acquiring charge. • If the
electroscope is negatively charged, the leaf divergence slowly
decreases indicating
that is losing charge. • If the electroscope is positively
charged, no loss of charge is observed. The
photoelectrons are attracted back to the zinc plate and
electroscope. Conclusion: The Zinc plate emits photoelectrons when
ultra violet radiation falls on it.
Vacuum Diode A diode is a device that allows the flow of current
in only one direction. A vacuum diode is one of such devices. It
works on the same principle as the C.R.O. It consists of an anode
and a cathode which is heated by a filament. All these are housed
in an evacuated glass envelope.
Zinc plate
Charged
electroscope
Ultra violet
lamp
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Action: The heater raises the temperature of the cathode, which
thermionically emits electrons. The electrons are accelerated to
the anode by the high p.d between the cathode and anode and
therefore a current I flows in the direction shown in the diagram.
If the supply is reversed, the diode does not conduct any current,
since the cool anode cannot now release electrons to flow the other
way to the cathode. This way the device acts as a rectifier. REVIEW
QNS ON PHOTOELECTRIC EFFECT (HOW TO PASS PHYSICS PAGE 258)
RECTIFICATION This is the conversion of alternating current to
direct current. A semiconductor diode rectifies, i.e. converts a.c.
to d.c. (a) Half wave rectification
The diode removes the negative half-cycles of a.c. to give a
varying but one-way (direct) p.d. across R, the ‘load’ requiring a
d.c. supply, Figure b above.
(b) Full wave rectification Both the half-cycles of the a.c to
be rectified are used. In the bridge circuit of fig. a, the current
flows the solid arrows when X is positive and Y negative and the
broken arrows on the negative half-cycles when the positive of X
and Y are reversed. During both half-cycles, current flows in R and
in the same direction giving a p.d. as shown below,
REVIEW QNS ON ELECTRONICS (HOW TO PASS PHYSICS PAGE 291)
THE END.